It has been observed that programs that work correctly for a SET of values in an EQUIVALENCE class fail on some special values. These values often lie on the BOUNDARY of the equivalence class. Boundary VALUE for each equivalence class, including the equivalence class of the output, should be covered. Boundary value test CASES are also called extreme cases. Hence, a boundary value test case is set of input data that lies on the edge or boundary of a class input data or that generates output that lies at the boundary of a class of output data.

It has been observed that programs that work correctly for a set of values in an equivalence class fail on some special values. These values often lie on the boundary of the equivalence class. Boundary value for each equivalence class, including the equivalence class of the output, should be covered. Boundary value test cases are also called extreme cases. Hence, a boundary value test case is set of input data that lies on the edge or boundary of a class input data or that generates output that lies at the boundary of a class of output data.